For a long time telephone network subscribers have been able to listen, dialing a suitable number, to different voice announcements (e.g. time, news, weather forecasts). Generally, these announcements are recorded on a continuous magnetic tape and are broadcast with continuous repetition, to several output units or translators. The subscriber can listen to the announcement only from the point which is being read at the instant at which he has access to the service. Thus the subscriber cannot generally listen to the announcement from the beginning and must remain connected until the announcement begins again and listen to it at least up to the point where he began the listening. This involves a time loss to the subscriber and a high degree of line occupation.
Furthermore, now subscribers tend to increasingly favor new kinds of announcements or, in general, new voice services (e.g. medical information, day-record . . . ) and some of the above services can require listening from the beginning. Then it becomes necessary to have equipment whose store is no longer a continuous magnetic tape. Such equipment must also allow the subscribers to have easy access to different announcements and services, and possibly guide the subscribers in the announcement and/or service choice. This entails considerable management difficulties, which make it suitable to provide computer-based equipments.
Some equipment of this kind has already been proposed, based on a digital announcement storage ("announcement" indicates here any information emitted in the voice band, including music).
Equipment of this type is described by R. J. Frank et al. and T. W. Anderson et al. in the articles "Mass announcement capability" and "Mass announcement subsystem", both appeared in The Bell System Technical Journal, V. 60, No. 6, July-August 1981 (pages 1049-1107).
In this equipment, several sub-systems store a certain number of announcements (or parts of them) coded in PCM at 64 Kbit/s. Each subsystem has two magnetic disks with 80 Mbyte capacity, divided into sectors corresponding to 30 s of voice. The two disks contain the same announcements and are read with a 15 s time shift, so as to present on a bus the same announcement every 15 s; the output network is synchronized with that announcement repetition rate.
This equipment presents some disadvantages, mainly depending on the system synchronous operation:
The announcement organization in 30 s periods hinders the storage of very short announcements, which must be repeated many times to fill the fixed duration; thus, guidance announcements which help the subscriber to accede different services cannot be stored. This facility would be desirable when a wide range of services is available.
Because of synchronous operation, the number of announcements which can be stored in each subsystem is limited (in particular, to 60 announcements), otherwise the average wait time for a subscriber, already quite high (7.5 s), would become intolerable. Similarly, the introduction of new announcements without increasing the wait time requires a change in the disk reading rate, i.e. a system modification.
Another disadvantage involves reliability and depends on the equipment structure, as in the case of failure of a subsystem the subscribers can no longer have access to the announcements stored therein.
Lastly, this known system has been conceived for connection to electronic exchanges and can supply announcements only to the subscribers connected to such exchanges.
A second type of system is described in the paper "Prospects for voice signalling in the telephone network" presented by D. S. Cheeseman and M. B. Cooper at 1982 International Zurich Seminar on Digital Communications, Zurich, Mar. 9-11, 1982 and published at pages 121 to 127 of the seminar proceedings.
This paper describes equipment in which the announcements to be forwarded to the subscribers, coded in delta modulation at 32 Kbit/s or in PCM modulation at 48 Kbit/s, are stored in a solid state store of 4 Mbit capacity. Each announcement consists, on the average, of 10 words, and about one hundred announcements, for a total vocabulary of 200 words. The store is connected to a bus at 32 Mbit/s to which 128 channels are connected and where each announcement is presented with a periodicity of 250-ms. This equipment is also synchronous.
This second system solves the problem of the excessive average wait time at the expense of the announcement length. In fact, it has been conceived to supply the subscribers with a speech guidance for the access to some services, but it cannot supply "information" announcements, unless they are very short. Moreover, the problem of serving subscribers connected to electromechanical exchanges is mentioned, but no solution for this problem is given.